The present invention relates generally to systems including filters and, more particularly, to a method and apparatus for monitoring the condition of a filter in a system including an air filter.
Disposable air filters are commonly used in commercial and residential heating, ventilating and air conditioning (HVAC) systems, and in portable air purification units. Such filters typically include a frame, a fibrous non-woven filter material, and a reinforcing structure that supports the filter material. After a period of use, these filters become dirty or clogged and must be replaced. Proper filter maintenance helps keep the HVAC equipment operating at maximum efficiency and reduces operating costs. Depending on the operating conditions, filter replacement may be required several times a year. Filter replacement is typically done on a regular, fixed-interval basis. That is, the filter is replaced after a certain recommended fixed period of time, such as three months, has passed. This fixed period of time, however, may not be appropriate for a given situation. For example, the fixed time period may be too short, in which case the filter is discarded prematurely, or the fixed time period may be too long, in which case the filter is used beyond the time when it should have been changed. Individuals may also lose track of when the filter was last changed or may forget to change the filter at the appointed time. Changing the filter prematurely is wasteful and results in unnecessary expense, and waiting too long can result in system inefficiency or may negatively impact air quality.
The pressure drop across a typical filter in a residential or commercial HVAC system, or in a portable room air purification unit, is usually quite low. For example, the pressure drop across a new filter may be about 0.2 inches of water (0.008 pounds per square inch—psi), and a filter in need of changing or cleaning may have a pressure drop of about 0.6 inches of water (0.024 psi). Devices used to measure pressures in this range are typically very delicate and expensive, and are generally ill-suited for use in consumer products.
Devices for indicating when to change or clean an air filter are known in the prior art. U.S. Pat. No. 4,321,070 (Bede), for example, discloses a whistle which, when installed in an air filter, emits a shrill tone to signal a predetermined clogged condition of the filter at which time the filter should be cleaned or replaced.
U.S. Pat. No. 3,071,914 (Gesmar) discloses a filter condition indicator for a filter fan using a differential pressure gauge. The differential pressure gauge includes a hollow indicator tube and a float that moves in response to the pressure differential. U.S. Pat. No. 5,668,535 (Hendrix et al) discloses a filter condition sensor and indicator in which a heated thermistor is positioned in a by-pass air flow path.
Residential HVAC systems and portable air purifiers often utilize high performance filters commonly referred to as panel filters or pleated filters. Such filters are available from 3M Company, St. Paul Minn., under the Filtrete™ brand. Such filters have a generally thin flat construction and include a somewhat rigid peripheral frame and an interior filtering region formed from a fibrous non-woven filter media. To improve the performance of such filters, the filter media may be electrostatically charged. Such filters come in a variety of sizes and may include a support structure formed of, for example, wire mesh, paperboard, chipboard or glue to provide additional support for the filter media. Even with such support structures, however, the interior filtering region of such filters tends to exhibit a certain degree of bending, flexing, bowing, or bulging when subjected to the load created by air passing through the filter.
Devices that detect a predetermined extent of filter budge to operate an end-of-life signal are also known in the patented prior art. U.S. Pat. No. 3,928,006 (Martineau) discloses a device for signaling dirty filters including a position sensitive electrical switch positioned to detect a predetermined extent of filter bulge, thereby to operate an end-of-filter-life signal. This device, however, suffers from certain limitations and drawbacks. For example, the operation of this device is highly sensitive to the position of the device relative to the filter, and therefore requires a mounting structure that allows the position of the device to be calibrated depending on the type of filter used and/or the operating conditions. In addition, the device is not capable of providing information about the condition of the filter continuously or periodically throughout the life of the filter.
There is still a need, however, for a system and/or device that can monitor the condition of a filter based on the degree of bowing or flexing of the filter. That is, there is a need for a system and/or device that can provide a person responsible for changing and/or cleaning a filter with information about the condition of the filter throughout the life of the filter including from the time a new clean filter is installed in the system and bows only minimally when subjected to an air stream, during the time when the filter is being used but is not yet in need of replacement and/or cleaning and the filter bows increasingly as the filter becomes more clogged, and up to the time when the filter becomes unacceptably clogged and is in need of replacement or cleaning.
It would be desirable to provide a small, simple, inexpensive system and/or device that can accurately and reliably monitor the condition of a filter, such as an air filter, either continuously or periodically over the entire life of the filter.